The overall goal of this project is to develop a safe and effective vaccine to prevent group A streptococcal (GAS) infections and their most serious complications, acute rheumatic fever (ARF), rheumatic heart disease (RHD) and invasive disease. We have made considerable progress in developing progressively complex recombinant multivalent M protein-based vaccines containing type-specific protective peptides from as many as 30 different M serotypes of GAS that account for 80-95% of infections in North America and Europe. Epidemiologic data from developing countries, where the burden of ARF/RHD is extraordinarily high, suggest that the potential efficacy of the 30-valent vaccine would only approach 50%. In our efforts to identify surface antigens of GAS that could be combined with M proteins to improve vaccine efficacy, we discovered that M- related protein (Mrp) of GAS evokes bactericidal antibodies against homologous and heterologous serotypes of GAS. Eighty-three percent of the 125 defined M types of GAS are Mrp-positive. Importantly, analysis of the Mrp sequences from 11 serotypes of GAS indicates a striking conservation of structure among three related families of Mrp, suggesting that cross-protective immunity may be achieved using a minimum number of protein fragments. Therefore, we have refined our approach to the development of a broadly protective vaccine by proposing to combine the current 30-valent M protein-based vaccine with cross-protective peptides of Mrp. In this application, we propose to: a) define the breadth of cross-protective immunity evoked by Mrp, b) determine the covalent structures of Mrp that contain cross-protective epitopes, and c) define the potential protective efficacy in animals of vaccines combining recombinant hybrid Mrp peptides with the 30-valent M protein-based vaccine. This project takes advantage of our experience using innovative approaches in the design and application of novel recombinant vaccine proteins, our ongoing studies to determine the epidemiology of GAS infections in Mali and South Africa, and our discovery that Mrp evokes serotype cross- protective immune responses in animals.